The present invention relates to a device for connecting an electric line to a connection terminal with a circuit breaker, and in particular to a connection device that in addition to enabling the electrical connection of the circuit breaker to an electric line, makes it possible to extract heat from the circuit breaker and transfer it to the electric line itself.
As known, low voltage breaking devices (that is for applications with nominal voltages up to 1000V AC/1500V DC), such as automatic circuit breakers, disconnectors, and contactors, commonly referred to as “switching devices” and hereinafter collectively referred to as circuit breakers, are devices designed for allowing correct operation of specific parts of electrical systems and installed loads.
Such devices are usually installed inside distribution switchboards located in electrical systems. Distribution switchboards normally comprise suitable cells or cubicles arranged for connecting the devices to the electrical power distribution lines. Distribution lines are normally constituted by systems of conductors, such as bus bars and/or cables. The use of appropriate distribution switchboards, in addition to improving practicality, ergonomics of use, and the aesthetic appearance of the systems, contributes to maintain over time adequate safety conditions and correct functionality of all installed parts.
The choice of the devices to be used and their installation methods thereof, should be compatible with the technical characteristics of the distribution switchboard. This compatibility relates to electrical, dimensional, mechanical, and thermal aspects. For circuit breakers, there are three main installation configurations in the switchboards.
In particular, a first installation method for circuit breakers is the so-called “fixed” execution wherein the electrical terminals of the circuit breaker are directly and stably connected to the conductors of the distribution lines. Such connection is normally done by using clamps or screws.
A second installation method for circuit breakers is the so-called plug-in execution wherein special adapter devices are used which are mechanically connected to the switchboard, and connected stably to the conductors of the distribution lines by means of their own electrical terminals; each circuit breaker is mechanically coupled to a corresponding adapter device and by means of appropriate plug-in electrical terminals, it realizes the electrical connection to the distribution line; plug-in coupling normally includes plug-socket type mechanisms.
A third installation method for the circuit breakers is the so-called withdrawable execution; it is substantially an evolution of the preceding removable method, wherein accessory elements are added such as guide and/or support and/or movement means for facilitating plugging and withdrawal operations of the circuit breaker.
Of these three installation methods, the first one is the simplest and cheapest, but it is only suitable to definitive solutions and in any case non-flexible; on the other hand, the removable and withdrawable-type methods offer a greater flexibility. These in fact allow (once the adapter is secured in the switchboard) very quick and totally safe installation or removal of the circuit breaker and, above all, without having to intervene directly on the distribution line.
Installations of circuit breakers of the removable and withdrawable-type do have at least one disadvantage with regard to the fixed-type installation. In order to realize plug-in junction (plug/socket), it is in fact necessary to introduce at least one additional electrical connector element. Considering the assembly composed by the circuit breaker and its related adapter, it is in fact possible to schematize each of its poles or branches as an electrical chain consisting of elements mutually placed in series. In such electrical chain, each element contributes to increase the electrical resistance (or analogously deteriorate the overall conductivity) and thus constitutes a potential source of heat due to the Joule effect.
The undesired heat is generated both in the various conducting sections (for example made of copper) and, above all, at each of the present electrical couplings. The various junctions present, and in particular the plug/socket plugs and the main contacts of the circuit breaker, which by their nature cannot be soldered, in fact introduce similar micro-discontinuities where conspicuous localized increases of electrical resistance can be found. In practice, the most critical energy dispersion peaks due to the Joule effect, with consequent undesirable heat production, tend to occur in these areas.
As can be seen, the heat that is generated due to these dispersions contributes to increase the temperature of the system consisting of circuit breaker, cubicle and switchboard. But since the temperature of the circuit breaker and the temperature of the switchboard should be maintained within predefined operating limits, any undesired increase of electrical resistance in the conducting branches of the system consisting of the circuit breaker and its related adapter compels limiting the power that can be drawn by an apparatus. In addition, the temperature can negatively influence the operation of the circuit breakers. It is likewise known that the temperature of the circuit breaker tends to increase more rapidly if the characteristics of the used adapter, of the cubicle, and of the switchboard favor the accumulation of heat. In practice, with appropriate computations, it is possible to define the maximum fraction of full theoretical load at which a circuit breaker can function in safe condition when it is installed in the cubicle of a switchboard. The fraction of the actually usable maximum load (with respect to the theoretical nominal capacity) is generally expressed in the form of “derating” coefficients that are based on the overall effective conditions of installation. These installation conditions take account of the combination of the characteristics of circuit breaker, adapters, cubicle, switchboard, external environment, etc.
Besides the constraints associated with derating, it is therefore desirable to maintain the operating temperature of the circuit breakers at low levels; it is well known in fact that the higher the operating temperature is, the lower the life span of the circuit breaker or of its more sensitive components.
Many solutions have been introduced by various manufacturers in order to reduce the electrical resistance of the poles of the circuit breakers and the electrical contact resistance of the electrical coupling between the circuit breaker and the adapter, and/or in order to improve the overall thermal efficiency of the switchboard.
Although these well-known solutions certainly provide some technical benefits, there is room and necessity for further improvements.